Power efficient stimulators

ABSTRACT

This disclosure relates to a device for applying a neural stimulus. A battery supplies electrical energy at a battery voltage and an electrode applies the electrical energy to neural tissue. A circuit measures the nervous response of the tissue and a voltage converter receives the electrical energy from the battery and controls a voltage applied to the electrode based on the measured nervous response of the tissue. This direct voltage control is energy efficient because losses across a typical current mirror are avoided. Further, the control based on the measured nervous response leads to automatic compensation of impedance variation due to in-growth or change in posture. As a result, the stimulation results in a desired neural response.

TECHNICAL FIELD

This disclosure relates to devices for applying a neural stimulus.

BACKGROUND

Neuromodulation is used to treat a variety of disorders includingchronic pain, Parkinson's disease, and migraine. A neuromodulationsystem applies an electrical pulse to tissue in order to generate atherapeutic effect. When used to relieve chronic pain, the electricalpulse is applied to the dorsal column (DC) of the spinal cord or dorsalroot ganglion (DRG).

Such a system typically comprises an implanted electrical pulsegenerator and a power source, such as a battery, that may berechargeable by transcutaneous inductive transfer. An electrode array isconnected to the pulse generator and is positioned in the dorsalepidural space above the dorsal column. An electrical pulse applied tothe dorsal column by an electrode causes the depolarisation of neuronsand generation of propagating action potentials. The fibres beingstimulated in this way inhibit the transmission of pain from thatsegment in the spinal cord to the brain.

While the clinical effect of spinal cord stimulation (SCS) is wellestablished, the precise mechanisms involved are poorly understood. TheDC is the target of the electrical stimulation as it contains theafferent A-beta fibres of interest. A-beta fibres mediate sensations oftouch, vibration and pressure from the skin. The prevailing view is thatSCS stimulates only a small number of A-beta fibres in the DC. The painrelief mechanisms of SCS are thought to include evoked antidromicactivity of A-beta fibres having an inhibitory effect and evokedorthodromic activity of A-beta fibres playing a role in painsuppression. It is also thought that SCS recruits A-beta nerve fibresprimarily in the DC with antidromic propagation of the evoked responsefrom the DC into the dorsal horn thought to synapse to wide dynamicrange neurons in an inhibitory manner.

Neuromodulation may also be used to stimulate efferent fibres, forexample to induce motor functions. In general, the electrical stimulusgenerated in a neuromodulation system triggers a neural action potentialwhich then has either an inhibitory or excitatory effect.

Effects can be inhibitory e.g. used to modulate an undesired processsuch as the transmission of pain, or stimulatory e.g. causing a desiredeffect such as the contraction of a muscle.

Spinal cord stimulators provide tissue stimulation using electrodes andcircuits to deliver electrical energy to the nervous tissue. They canuse charge balanced biphasic pulses or monophasic pulses with resistorsand capacitors for charge recovery. Some stimulators use tri-phasicstimulation.

FIG. 1 illustrates a spinal cord stimulator using an example power path.A battery 101, typically 3.25-4.2V provides power to a switch-mode powersupply 102 that pumps the voltage to a supply called VddHV, at typically8-15V. A current mirror (P1, P2) 103 controlled by a reference current104 creates a controlled current which then flows through switches 105,106, 107, 108 to tissue 109. The switches 105, 106, 107, 108 arearranged in an H-bridge allowing current to be driven using eitherpolarity into the tissue. With the switches 105, 106, 107, 108 in theposition shown, charge flows from left to right. A shorting switchallows unbalanced charge to be recovered. Alternately this can beachieved by closing switch 107 and switch 108 together. Capacitors 110and 111 block DC current from flowing in tissue 109. By using two suchcapacitors, no DC current can flow even when one capacitor fails. Thevoltage across the tissue 109 can be as high as 14V, so VddHV values of16.5V are common.

It takes power to drive current into tissue. This power is drawn fromthe battery 101 and drains it so that it must be recharged regularly.Recharging the battery is an inconvenience to the patient. It isdesirable to build a stimulator that is as efficient as possible, whilenot changing stimulation strength as the patient changes posture.

With current drive, the battery is pumped up to VddHV which exceeds themaximum induced tissue voltage by at least 0.5V, which is used to biasthe current driver transistors. This ‘lost’ voltage is marked asV_(loss) 112 and can be expressed as Vloss=VddHV−Vload andPloss=Iload(VddHV−Vload). Power is dissipated in the implant when thecurrent flows through the transistors of current source 103.

The power lost can also be given by the following equation where V_(L)is the lost voltage and I is the stimulation current.

P=V_(L)I

Since a single value of VddHV is often chosen, when the patient has thestimulation strength turned low, the power lost in the drive transistorscan exceed the power delivered to the patient. It is clearly desirableto reduce this lost power to maximize battery life and so improvepatient convenience. In general, switched-mode power supplies obeyconservation of power, with the current and voltage on input and outputbeing related (ignoring the switcher efficiency term) by:

P=V _(DDHv) ·I _(PATIENT) =V _(BATTERY) ·I _(BATTERY)

It is thus recognized that if V_(DDHv) is pumped to 16V, for example,but the patient tissue only requires 4V, 75% of energy drawn from thebattery is wasted. If the rest of the implant were to be designed to useless power than this (as is desirable) then the time between batteryrecharges is potentially 4 times shorter than it need be.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is not to betaken as an admission that any or all of these matters form part of theprior art base or were common general knowledge in the field relevant tothe present disclosure as it existed before the priority date of each ofthe appended claims.

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

SUMMARY

A device for applying a neural stimulus comprises:

a battery to supply electrical energy at a battery voltage;

an electrode to apply the electrical energy to neural tissue;

a circuit to measure the nervous response of the tissue; and

a voltage converter to receive the electrical energy from the batteryand to control a voltage applied to the electrode based on the measurednervous response of the tissue.

It is an advantage that the voltage converter controls the voltageapplied to the electrode. In contrast to current control this directvoltage control is more energy efficient because losses across a typicalcurrent mirror is avoided. A further advantage is that the control basedon the measured nervous response leads to automatic compensation ofimpedance variation due to in-growth or change in posture. As a result,the stimulation results in a desired neural response, which previouslyrequired current control with the associated low energy efficiency.

The converter may comprise a processor programmed to calculate a voltagevalue based on the measured nervous response and to generate a controlsignal to the voltage converter indicative of the calculated voltagevalue.

The converter circuit may be a linear voltage-to-voltage converter.

The converter may be a switched-mode voltage to voltage converter.

The converter may comprises a pulse generator configured to generate apulse signal to control switching of the switched-mode voltage tovoltage converter.

The pulse signal may be based on the measured nervous response of thetissue.

The pulse generator may be a digital processor.

The device may comprise an analog-to-digital converter to provide adigital signal indicative of the measured nervous response of the tissueto the digital processor.

The voltage to voltage converter may comprise a switch that iscontrolled by a control signal based on the measured nervous response ofthe tissue to thereby control the voltage applied to the electrode basedon the measured nervous response of the tissue.

The control signal may define a duty cycle based on the nervous responseof the tissue, such that the control signal controls the switch and theduty cycle defines the output voltage to thereby control the voltageapplied to the electrode based on the measured nervous response of thetissue.

The control signal may be an analog voltage signal provided by theprocessor and the voltage signal controls the switching of the switch tothereby control the voltage applied to the electrode based on themeasured nervous response of the tissue.

The controller may comprise an oscillator with an oscillation frequencyand the voltage signal controls the oscillation frequency to therebycontrol the voltage applied to the electrode based on the measurednervous response of the tissue.

A method for neural stimulation comprises repeatedly performing thesteps of:

generating a stimulation voltage signal at a stimulation voltage;

applying the stimulation voltage signal to neural tissue;

measuring a nervous response of the tissue; and

adjusting the stimulation voltage based on the measured nervousresponse.

Generating the stimulation voltage may comprises repeatedly switching aswitched mode power supply; and adjusting the stimulation voltage maycomprise adjusting a pulse signal that controls the switching.

A device for applying a neural stimulus comprises:

a battery to supply electrical energy at a battery voltage;

an electrode to apply the electrical energy to neural tissue;

a circuit to measure the nervous response of the tissue;

a current mirror to deliver a current to the electrode according to areference current that is based on the measured nervous response; and

a voltage converter to receive the electrical energy from the batteryand to control a voltage applied to the current mirror based on avoltage between the stimulating electrodes.

It is an advantage that the voltage converter controls the voltageapplied to the current mirror based on the voltage between theelectrodes. This means the voltage applied to the current mirror can bereduced to reduce the voltage drop across the current mirror and therebyreduce the power dissipated in the current mirror.

The converter may be a switched-mode voltage converter.

A device for applying a neural stimulus comprises:

a battery to supply electrical energy at a battery voltage;

an electrode to apply the electrical energy to neural tissue;

a circuit to measure the nervous response of the tissue;

a switched mode voltage to current converter to receive the electricalenergy from the battery and to control a current applied to thestimulating electrode; and

a controller to control switching of the switched mode voltage converterbased on the measured nervous response of the tissue.

The controller may control the switching based on the battery voltage.

The controller may control the switching based on an electrode voltage

The controller may control the switching based on a desired stimulationintensity.

The controller may comprise a pulse generator to generate a pulse signalto control the switching.

The controller may comprise a voltage controlled oscillator to generatethe pulse signal.

The controller may comprise a voltage controlled delay controlled by thebattery voltage to control the switch.

The voltage controlled delay may be connected to a switch to disconnectan inductance from the battery after a delay controlled by the batteryvoltage.

The voltage controlled delay may be connected to the switch todisconnect the inductance from the battery after a delay controlled by atissue voltage.

The voltage controlled delay may be connected to the switch todisconnect the inductance from the battery after a delay controlled by adesired level of stimulation intensity.

The controller may comprise a voltage controlled oscillator to control afrequency of the pulse signal based on a desired level of stimulationand tissue voltage and a voltage controlled delay to control a timeperiod for which the switch connects the inductance to the battery ateach oscillation based on the battery voltage.

The pulse signal may be periodic and controlling the switch comprisessuppressing pulses that turn the switch on to set the amount of energyprovided by the inductance.

A device for applying a neural stimulus comprises:

a battery to supply electrical energy at a battery voltage;

an electrode to apply the electrical energy to neural tissue;

a circuit to measure a nervous response of the neural tissue;

a pulse generator to generate stimulation current pulses at a pulselength and to adjust the pulse length based on the measured nervousresponse of the neural tissue.

The circuit to measure the nervous response of the neural tissue maycomprise a template signal and the circuit is configured to shift thetemplate signal in time relative to the stimulation current pulses basedon the pulse width.

The circuit may comprise a look-up table storing delay values for thetemplate signal for each of multiple pulse width values.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a spinal cord stimulator using an example power path.

An example will now be described with reference to the followingdrawings:

FIG. 2a illustrates the resulting voltage and current waveforms forvoltage stimulation.

FIG. 2b illustrates the voltage and current waveforms for currentstimulation.

FIG. 3 shows a general architecture 300 of a stimulator with feedback.

FIG. 4 illustrates an example array of electrodes.

FIG. 5 illustrates a signal flow for processing the output signal fromamplifier 405 in FIG. 4.

FIG. 6a illustrates an example device for applying a neural stimulus.

FIG. 6b illustrates an example linear voltage-to-current controller.

FIG. 7 illustrates another example device for applying a neuralstimulus.

FIG. 8 illustrates a variation of the device in FIG. 7 where anadditional transistor is included.

FIG. 9 illustrates a further example device for applying a neuralstimulus.

FIG. 10 illustrates converter 906 from FIG. 9 in more detail.

FIG. 11 illustrates the operation switch 1003 from FIG. 10 in thecontext of neural stimulation.

FIG. 12 illustrates a time delay circuit.

FIG. 13 illustrates a digital controlled resistor circuit.

FIG. 14 illustrates a control circuit 1400 which implements the feedbackcircuit 904 in FIG. 9.

FIG. 15 illustrates a switched mode voltage converter with cascodep-channel FET in the output path.

FIGS. 16 and 17 show examples of a digitally controlled resistance orconductance, respectively.

FIG. 18 illustrates a deterministic way of combining two amplitudecontrols in a pulse modulation system.

FIG. 19 illustrates another example for a device 1900 for applying aneural stimulus.

FIG. 20 illustrates a stimulus and the alignment between ECAP andtemplate.

FIG. 21 illustrates a further example where the feedback control isimplemented directly in the converter.

FIG. 22 illustrates a further example stimulation device.

FIG. 23 illustrates a voltage-drive version of FIG. 22.

FIG. 24 illustrates a further implementation of an implantablestimulation device where, compared to FIG. 23, the DAC is omitted andthe controller directly provides the timed pulses for the switching inswitched-mode power supply converter.

FIG. 25 illustrates a method for neural stimulation.

DESCRIPTION OF EMBODIMENTS

In many cases current drive is preferred by patients as they find widepulse widths more ‘ soothing’. Due to the reactive nature of the tissueelectrode interface, when tissue is driven with a voltage source, thecurrent has a large spike at the beginning, then tails off. FIG. 2aillustrates the resulting voltage and current waveforms for voltagestimulation. FIG. 2b on the other side, illustrates the voltage andcurrent waveforms for current stimulation. The voltage stimulation isakin to a narrow stimulation pulse. In contrast, with current drive, awide rectangular stimulation can be produced which is preferred.

This disclosure will focus on systems using biphasic stimulation,although methods described can be adapted to greater or lesser numbersof phases. It will also describe both voltage and current sourcesystems.

FIG. 3 shows a general architecture 300 of a stimulator. An amplifier301 amplifies a physiological signal that, for an evoked compound actionpotential (ECAP), will typically be 10 uV to 100 uV. Its amplitude isthen detected using a variety of methods, though a correlator and 4-lobedetector is preferred. FIG. 4 illustrates an example array of electrodes401, which are spaced apart by 7 mm. At a typical propagation velocityof the ECAP of 60 m/s, the travel time between two adjacent electrodesis 116 μs. FIG. 4 also shows the ECAP waveform 402 at the same scale asthe electrode array 401 to illustrate the wavelength against the size ofthe electrode. At the currently illustrated point in time, the ninthelectrode 403 measures the P1 peak of the ECAP and the fifth electrode404 measures the N1 peak. A differential amplifier 405 amplifies thedifference between two electrodes which results in a filter for the ECAPsignal. The distance between two electrodes that are connected to thedifferential amplifier 405 may be between 14 mm and 42 mm, which relatesto between 2 and 6 electrodes for a 7 mm spacing between adjacentelectrodes.

FIG. 5 illustrates a signal flow for processing the output signal fromamplifier 405 in FIG. 4, which is now input signal 501. A first mixer502 mixes a reference sine wave 503 with a window function 504 and again module 505 scales the result by a weighting factor. A second mixer506 mixes the scaled result with the input signal 501. A summationmodule 507 sums up the samples over the time window of the windowfunction 504 similar to a continuous integral and provides an outputsignal 508. As a result of the mixing, the summed output reflects thesimilarity between the input signal and the windowed sine function,which is also referred to as a template function. That is, when the P1peak of the ECAP signal coincides with the maximum of the sine function,the output 508 has a maximum value. When the input signal 501 ismisaligned or contains mainly noise, the output 508 is minimal. Oneexample of this correlation process is a four-lobe correlation where thelength of the window function 504 spans four extrema of the sine wave,that is, the length of the window function 504 is twice the period ofreference sine wave 503.

The described process is similar to a correlation function between twosignals where one signal is time-shifted and integrated for each valueof the correlation function. For this reason, the described process isalso referred to as a correlation process and suppresses noise andartefacts such that the maximum of the correlation signal 508 can beused as a feedback value in the controls disclosed herein. The templatecan be time-aligned with the expected ECAP curve by calculating anexpected time of arrival, which depends on the distance from thestimulating electrode assuming t=0 from the start of the cathodic(negative) pulse, where the ECAP begins to propagate. For example, ittakes 467 us for the ECAP to travel to an electrode 28 mm from thestimulation site and PW is 120 us, then the sample delay is: 467−PW=347us. Time of arrival can also be simply measured. Further details of ECAPmeasurement are provided in WO 2014/071445 and WO 2014/071446, which areboth included herein by reference.

Once the evoked response amplitude has been calculated, such as thevalue of the correlation, a comparator compares the amplitude of thedetected evoked response with the desired response. A controllerintegrates the error signal at a rate that sets the loop time constantand a stimulator then generates a controlled stimulus pulse. Either theamplitude or the pulse width may be controlled.

Voltage Drive with Feedback

FIG. 6a illustrates an example device 600 for applying a neural stimulusas described above. The device 600 comprises a battery 601 to supplyelectrical energy at a battery voltage 602 and an electrode 603 thatapplies the electrical energy to neural tissue (not shown). Device 600further comprises a circuit 604 that measures the nervous response ofthe tissue, such as the ECAP described above. Further, there is avoltage converter 605 that receives the electrical energy from thebattery and controls a voltage applied to the electrode based on thenervous response of the tissue measured by circuit 604. As shown in FIG.6 the ECAP may be measured by a sense electrode 606, which may belocated at a distance from the stimulation electrode 603 as describedabove with reference to FIG. 4. This way the sense electrode 606 cancapture the evoked neural response once it has travelled the distancebetween the output electrode 603 and the sense electrode 606. To avoiderror measurements, the ECAP detection is disabled during thestimulation itself and shortly thereafter. This reduces artefacts causedby the settling of the circuitry, such as operational amplifiers.

It is noted that when the stimulator 600 in FIG. 6a is compared to priorart stimulator 100 in FIG. 1, the voltage converter 605 is connecteddirectly to the output electrode 603 without the use of a currentdriving circuit, such as current mirror 103. The power-saving stimulatordesign presented in FIG. 6 combines voltage drive and local feedback.The feedback compensates for the changes in stimulation current withtissue growth and posture. As the tissue is driven directly, the powerthat is dissipated by current mirror 103 (as shown in FIG. 1) is saved,which means the battery 601 lasts longer without being re-charged. Atthe same time, the feedback 604 of the evoked response allows thecontrol of the output voltage such that a desired response can bemaintained, which would previously have been achieved by current mirror103 without feedback.

In one example, the converter 605 comprises digital circuitry, such as amicroprocessor, in contrast to analogue circuitry, such as operationalamplifiers and current mirrors. In the digital case, the microprocessorcalculates a voltage value that is to be applied to the electrode 603.The voltage value may be in the form of an binary number, such as an 8bit string. An digital to analogue converter can then convert the bitstring into a voltage and delivered to electrode 603 through a drivercircuit. The processor may have stored on memory a desired value ofneural stimulation, which can be adjusted externally by the patient orthe clinician. In that case, the processor receives the measured ECAPfrom circuit 604 and compares the received ECAP with the stored desiredECAP. If the received ECAP is less than the desired ECAP, the processorincreases the voltage. On the other hand, if the received ECAP isgreater than the desired ECAP, the processor decreases the voltage. Theprocessor may also implement a proportional/integral/differential (PID)control mechanism which optimally responds to changes in the ECAP. Theinput (process variable) of the PID control is the measured ECAP whilethe error value is the difference of the input to the stored desiredECAP and the output is the electrode voltage or an output signal thatdirectly controls the electrode voltage. This can be useful if thepatient moves and the impedance of the electrodes changes or moregenerally the evoked response changes for a given electrode voltage. ThePID control loop can be parameterised for different objectives, such asfast response or minimal overshoot to avoid patient discomfort. Thegeneral PID calculation is given by

${{u(t)} = {{K_{p}{e(t)}} + {K_{i}{\int_{0}^{t}{{e(t)}dt}}} + {K_{d}\frac{d{e(t)}}{dt}}}}.$

In another example, the voltage converter 605 comprises a linearvoltage-to-voltage converter also referred to as linear voltageregulator. In such a case, the processor provides an output signal tothe linear voltage-to-voltage converter to control the linearvoltage-to-voltage converter to adjust the voltage as indicated by thePID control method. FIG. 6b illustrates an example where processor 606provides a control signal 607 to an operational amplifier 608 thatdrives the gate of an output transistor 609. This means the outputvoltage of the processor 606 or DAC connected to the processor 606constitutes the output value of the PID control. It is noted that thelinearity of the regulator, such as the output transistor 609, is notcrucially important because the PID control automatically adjusts thevoltage by relatively small variations and a slight non-linearity shouldnot affect the operation of the overall control loop.

In yet another example, the voltage converter 605 in FIG. 6a comprises aswitched-mode voltage to voltage converter. In this case, the processorcontrols the duty cycle of the switching, that is, the processor variesthe ratio of on-to-off time. During the on-time an inductor is chargedwhile during off-time the energy stored in the inductor is consumed bythe electrodes. According to the principle of switched-mode voltageconverters, a higher duty cycle increases the output voltage while alower duty cycle reduces the output voltage. Consequently, the processorcan control the output voltage by controlling the duty cycle.

Current Drive with Local Feedback

FIG. 7 illustrates another example device 700 for applying a neuralstimulus. Device 700 comprises a battery 701 to supply electrical energyat a battery voltage 702 and an electrode 703 to apply the electricalenergy to neural tissue. A circuit 704 connected to a sense electrode705 measures the nervous response of the tissue. In this case, thedevice performs current drive and therefore comprises a current mirror706 to deliver a current to the electrode 703. A reference currentsource 707 provides a constant reference current to current mirror 706,which can be adjusted by the clinician or the patient to adjust thelevel of perceived effect of the stimulation. The reference currentsource 707 is also controlled by the neural response measured by circuit704. Current mirror 706 then mirrors the reference current from source707 and delivers it to the output electrode 703. As a result, thecurrent delivered to the output electrode is based on the measurednervous response in the sense that a lower response leads to anincreased reference current and a higher response leads to a lowerreference current. Again, the circuit 704 may include a processor andthe processor may perform PID control to generate the signal thatcontrols the reference current source 707.

Importantly, a voltage converter 708 receives the electrical energy fromthe battery and controls a voltage applied to the current mirror basedon a voltage between the stimulating electrodes. This means the voltageapplied to the current mirror can be reduced to reduce the voltage dropacross the current mirror and thereby reduce the power dissipated in thecurrent mirror. There is also an H-Bridge 709 to switch the outputcurrent to the output electrode 703.

In the example of FIG. 7 device 700 comprises an operational amplifier710 (i.e. differential amplifier) that provides the feedback signal toconverter 708. The inputs of amplifier 710 are connected to the outputelectrode 703 and the drain 711 of a first transistor 712 ofsub-regulator 706, which also comprises a second transistor 713. Sincethe gates of the first transistor 712 and the second transistor 713 areconnected, the gate source voltage is identical leading to approximatelyidentical drain currents according to the principles of current mirrors.The difference between the gate voltage and the electrode voltage isthen the gate drain voltage of the second transistor 713. The aim shouldbe to keep this voltage to a minimum to reduce the power dissipated inthe second transistor 713. Therefore, the output of amplifier 710 isconnected as a control input to voltage converter 708 such that thevoltage converter 708 reduces its output voltage when there is a largegate drain voltage across the second transistor. In other words, if alarge voltage is created on stimulation electrode 703, the gate-drainvoltage across second transistor 713 will be low and converter 708 willkeep or increase its output voltage. On the other hand, if a low voltageis created on stimulation electrode 703 (due to lower tissue impedance,for example) the gate drain voltage across second transistor 713 islarger, which causes converter 708 to decrease its output voltagethereby reducing losses across second transistor 713.

The V_(loss) term is kept to one transistor turn-on voltage and powerloss is reduced. The drain-source voltage of second transistor 713 isjust sufficient for second transistor 713 to be saturated, where asimple mirror operates it with a drain voltage equal to the saturationvoltage plus a threshold. For a typical CMOS process with thresholdvoltages of 0.5V, additional improvement can be obtained by biasingsecond transistor 713 closer to its saturation limit as shown in FIG. 8where an additional transistor 801 is included between first transistor712 and the reference current 707.

In one example, converter 708 is a switched-mode voltage converter wherethe duty cycle of charging the internal inductance depends on the outputsignal of amplifier 710.

FIG. 9 illustrates a further example device 900 for applying a neuralstimulus. Device 900 comprises a battery 901 to supply electrical energyat a battery voltage 902. Device 900 further comprises an electrode 903to apply the electrical energy to neural tissue and a circuit 904connected to a sense electrode 905 to measure the nervous response ofthe tissue. There is also a switched mode voltage to current converter906 to receive the electrical energy from battery 901 and to control acurrent applied to the stimulating electrode 903. Importantly, there isno current mirror in FIG. 9 because the switched mode converter 906provides the current directly to output electrode 903. Circuit 904 isconnected to converter 906 to control switching of the switched modevoltage converter based on the measured nervous response of the tissue.In other words, this takes the feedback control 904 directly into theconverter 906, and avoids the use of a sub-regulator 706 in FIGS. 7 and8. The solution described in FIG. 9 and the related subsequent Figuresprovide most of the benefits of voltage drive, while fitting within theframework of existing clinical systems. This means there is provided away to build a current source for tissue stimulation that uses lesspower than previous implementations.

Current source stimulators typically provide current over a range from50 uA to 12.5 mA and should be selectable. Device 900 provides a fixedpulse width that is stable from one stimulation cycle to the next. As aresult, the ECAP appears at a predictable time and can be detected. Thepulse width is usually adjusted by the clinician to a value that ispreferable to a patient. The battery voltage changes as the battery isdischarged and the tissue voltage changes during the stimulation pulse.The current mirror 103 in FIG. 1 addresses these issues but the problemof power loss remains.

FIG. 10 illustrates converter 906 in more detail, which comprises acharge pump to multiply the battery voltage 902 to the higher voltageused to stimulate the tissue. It is noted that the charge pump 906 isconnected directly to tissue 1001. Importantly, converter 906 comprisesan inductor 1002 and a switch 1003 that closes a circuit including onlyinductor 1002 and battery 901. As a result, a current will flow throughinductor 1002 and switch 1003. This current will be low due to theself-inductance of inductor 1002 and then rise as the magnetic fieldbuilds up. According to the principles of inductors, when switch 1003 isopened the current through inductor 1002 remains constant, which meansthat the voltage can increase above the battery voltage if the connectedresistance is high. This value of the voltage depends on the energystored in inductor 1002 and therefore on how long switch 1003 was closedbefore it was opened. A capacitor 1004 smooths the voltage to largelyeliminate any spikes from switching and a diode 1005 avoids reversecurrent from the capacitor 1004 into the inductor 1002. A Zener diode1006 provides over voltage protection. This stops the output voltagefrom going too high and damaging components that have a stress limite.g. when the load is accidentally disconnected. In this case this diodeis connected to a small resistor R_(P) 1007.

FIG. 11 illustrates the operation switch 1003 in the context of neuralstimulation. Switch 1003 connects inductor 1002 to battery 901 for timet₁. Switch 1003 then connects inductor 1002 to tissue 1001 and currentflows for a time t₂. Pulses are generated with a period t₃ and last fora period equal to the stimulus pulse width PW. The time t₁ controls howmuch energy is stored in the inductor. The problem of designing a usefulcurrent source consists of controlling these times in a useful manner.It is understood the time PW is simply the time the current source isenabled. The method to generate t₁, t₂ and t₃ is described below.

Once connected to the load, the inductor will pump charge into the loaduntil it has no more energy to do so, and then due to the presence ofdiode 1005 the current will cease in a self-regulating manner. So thetime t₂ is self-regulating.

Energy Equations

To appreciate how to control t₁, t₂ and t₃ in light of the requirementspreviously provided, it is useful to derive the equations of theswitched mode charge pump of FIG. 10.

Assuming the charge pump cycle begins with zero current in inductor1002, its current is given by:

$I = \frac{Vt}{L}$

where the voltage source voltage is V, the time the inductor isconnected is t and the inductance is L . The identical equationdescribes the time taken for the inductor to dump all its power into aload, ending with the current in the inductor being zero. Thus, thisequation applies to FIG. 10 with the switch in either position.

The energy stored by the inductor is

$E = {\frac{L}{2}I^{2}}$

Substituting

$E = {{\frac{L}{2}\frac{V^{2}t^{2}}{L^{2}}} = {\frac{1}{2L}V^{2}t^{2}}}$

Since energy is the product of power and time, and charge is the productof current and time:

E=V₁I₁t₁=V₁Q₁=V₂I₂t₂=V₂Q₂

Although the time t₁ can be controlled, the time t₂ then depends on V₁and V₂ in order to obey conservation of energy.

The charge delivered each cycle is:

$Q_{2} = {\frac{E_{2}}{V_{2}} = {\frac{V_{1}^{2}t_{1}^{2}}{2L}\frac{1}{V_{2}}}}$

And the average current delivered is

${I = {\frac{E_{2}}{V_{2}} = \frac{V_{1}^{2}t_{1}^{2}}{2L}}}\frac{f}{V_{2}}$

Since t₃ is the reciprocal of f:

${I = {\frac{E_{2}}{V_{2}} = \frac{V_{1}^{2}t_{1}^{2}}{2L}}}\frac{1}{t_{3}V_{2}}$

Since the charge delivered depends on t₁, t₂, t₃, V₁ and V₂, it isuseful to provide a predictable average current and to eliminate thesedependencies.

Circuitry

FIG. 12 illustrates a circuit component 1200 that can be used to achievethis in the sense that circuit component 1200 creates a time delayinversely proportional to a control voltage. This is called a “voltagecontrolled delay” (VCD) circuit and in this form it has two controlswith the delay being proportional to the ratio of the two voltages.Circuit component 1200 comprises a capacitor 1201 that is dischargedthrough a discharge transistor 1202 when a trigger signal 1203 connectedto the base of discharge transistor 1202 goes high. A current mirror1206 charges the capacitor 1201 by mirroring a reference current 1207when the trigger signal 1203 goes low. This reference current 1207, inturn, is controlled by the second voltage V_(IV) 1208 amplified byamplifier 1209 which is buffered by buffer transistor 1210 including anegative feedback that ensures that the emitter voltage (i.e. thevoltage across resistor 1211) is equal to V_(I) 1208. As a result,reference current 1207 is

${I_{ref} = \frac{V_{I}}{R}}.$

In essence, if V_(I) 1208 is high, a high reference current flows throwresistor 1211, leading to a high current into capacitor 1201 and ashorter time for charging capacitor 1201. Therefore, the delay for arising edge is inversely related to V_(I). The delay for a falling edgeis determined by dimensions of transistor 1202, which can be chosen suchthat the delay for the falling edge is relatively short. In other wordsthe falling edge is substantially instantaneous with a negligible delaycaused by discharge transistor 1202. Conversely, a lower second voltageV_(I) 1208 leads to a longer delay of the rising edge because the timeto charge capacitor 1201 is longer. On the other hand, a high voltagefor Vp 1205 leads to a longer delay since the voltage across capacitor1201 needs to rise further before the output goes high. More formally,the general current voltage relation for capacitor 1201 is

${I = {C\frac{d\; V}{dt}}},{{{so}\mspace{14mu} {dt}} = {C{\frac{d\; V}{I}.}}}$

Substituting the (constant) reference current yields

${dt} = {RC{\frac{d\; V}{V_{I}}.}}$

Considering that the required voltage difference to cause the output toswitch is dV=V_(P), the time from the trigger pulse going low to theresponse signal going high is

$t = {RC\frac{V_{P}}{V_{I}}}$

In this, V_(P) is considered the proportional control voltage and V_(I)is the inverse control voltage.

In order to use component 1200 to generate the time t₁ the batteryvoltage is used as the inverse control V_(I) 1208 and the proportionalcontrol voltage is kept constant. The result is for some constant a:

a=V₁t₁

with a=RCV_(P). The variation of energy in the inductor due to thevarying battery voltage is hence eliminated.

To control the average current the situation is more complicated. It isdesirable to increase the inductor energy to compensate for thedecreased charge that is delivered as the load voltage increases. At thesame time it is necessary to provide current control for the clinician,patient and the control loop. This control signal is digital.

FIG. 13 illustrates a digital controlled resistor circuit 1300comprising four resistors 1301, 1302, 1303, 1304 and correspondingswitches 1311, 1312, 1313, 1314. In this example, each resistor has aresistance that is double the resistance of the next smaller resistorsimilar to a binary number system. Each switch is controlled by one bitin a digital control signal M 1320. As a result, the overall resistanceof the resistor circuit 1300 is set by the digital signal M 1320 suchthat each bit in M reduces the overall resistance by adding a parallelpath. When resistor 1211 in FIG. 12 is now replaced by controlledresistor circuit 1300, the reference current 1207 increases with eachactive bit in M, which in turn decreases the charge time of capacitor1201 decreasing delay t, which leads to a shorter charge time ofinductor 1002 in FIG. 10 which finally leads to a decreased stimulationcurrent.

Since the resistance is a multiplicative term in the expression above,the resulting circuit with controlled resistor circuit 1300 replacingresistor 1211 is referred to as “multiplying VCD” (MVCD) whichmultiplies the compensating term from the load voltage and the digitalcontrol. So, the MVCD has three inputs, Vi, Vp and M.

FIG. 14 illustrates a control circuit 1400 which implements the feedbackcircuit 904 in FIG. 9. The control circuit 1400 comprises an MVCD 1401as shown in FIG. 12 but with the digitally controlled resistor circuit1300 from FIG. 13 in place of resistor 1211. Control circuit 1401further comprises a VCD 1402 as shown in FIG. 12. A feedback loop 1403including an inverter 1404 causes an intermediate signal 1405 tooscillate and the oscillation frequency depends on the delay created byMVCD 1401 and generates the time t₃ where the pump output V₂ (voltageacross switch 1003 in FIG. 10) is connected as the inverse controlvoltage V_(I). VCD 1402 is used with V₁ (battery voltage) as its inversecontrol, we can write, for some constant b of the second VCD circuit:

${t_{3} = {b\frac{V_{1}}{V_{2}}}}.$

Then

$I = {\frac{a^{2}}{2L} \cdot \frac{V_{C}}{b}}$

At this point we have a current source that is controllable. The timebetween rising edges is controlled by

Out of Compliance Circuit

It is desirable that a clinician can detect when a current source goesout of compliance. In this case, this occurs when the shunt voltageregulator 1006 of FIG. 10 starts to conduct. A monitor on the resistorR_(P) 1007 achieves this.

Design Range of First VCD

In the case where the battery voltage varies from 4.2V to 3.25V (atypical range for a lithium-ion rechargeable cell) the value of t₁varies over a range of 1.29:1. This is a small range and so the designof the t₁ VCD is not problematic. This leaves room for additionalcontrol for the overall feedback and clinician control.

The V_(P) inputs to the two VCDs are unused. They could be controlled byDACs to provide different current ranges. The range from 50 uA to 12.5mA varies by 1:250. The load can vary from 1V to 15V, so the totalvariation is greater than 1:3750. If the PW=100 us, then the requiredresolution is 26 ns. This is technically difficult. The V_(p) inputsprovide additional degrees of freedom to span this space.

Design Range of Second VCD

A solution to this problem is to waste a bit of voltage in the load asshown in FIG. 15. The cascode p-channel FET 1501 limits the load voltageto the sum of the voltage V_(L) plus the p-FET turn on voltage 0.6V, atsmall values of the load impedance or small currents. At higherimpedances, the load voltage becomes larger and in the limit the FETdrain-source voltage tends to zero and the FET becomes a small parasiticresistance in the current delivery chain. This modification will limitthe battery life improvements for patients who have comparatively lowtissue impedances, but there will still be considerable improvementcompared to the alternative of dissipating power in the current sourcetransistor.

Depending on the load, the voltage V₂ can vary between the maximum thecircuitry can withstand and where the Zener diode turns on (at say16.5V) to the sum of the p-FET source voltage when there is a zero ohmload plus the diode voltage. Since V_(L) is under the control of thedesigner this can be arbitrarily chosen; a value of 5V would besuitable. In this instance the voltage V₂ would vary from 16.5V to 5Vi.e. a range of 3.3. Again, there is room to incorporate additionalcontrol.

FIGS. 16 and 17 show examples of a digitally controlled resistance orconductance, respectively. Observing that FIG. 14 has two resistors (onein MVCD 1401 and one in VCD 1402) and either can be a controlledresistance or a controlled conductance, and one appears in the numeratorand one in the denominator of the current equation, then there are a lotof options. Now these can be placed in the R spot, or driven with acurrent source to provide the proportional and inverse inputs.

The DAC

FIG. 18 illustrates a deterministic way of combining two amplitudecontrols in a pulse modulation system. It is also possible to use randomvalues as per a delta-sigma DAC.

Example Numeric Values

Inverting the equation

${I = {\frac{E_{2}}{V_{2}} = \frac{V_{1}^{2}t_{1}^{2}}{2L}}}\frac{1}{t_{3}V_{2}}$

we get

${L = {\frac{E_{2}}{V_{2}} = \frac{V_{1}^{2}t_{1}^{2}}{2I}}}\frac{1}{t_{3}V_{2}}$

Substituting V₁=3.25, t₁=500 ns , I=12.5 mA, t₃1 us , V=15V gives L=7uH.

This provides 1 us per pulse (0.5 us to charge the inductor, 0.5 us todump it), so in a 100 us stimulus pulse, we have about 6.5 bits ofcontrol. However, a feedback term and clinician term may need to beincluded.

FIG. 19 illustrates another example for a device 1900 for applying aneural stimulus. In this example, device 1900 comprises a battery 1901to supply electrical energy at a battery voltage and a fixed currentsource 1902 powered by battery 1901. The fixed current source 1902 isfixed in the sense that it is set at a relatively high current. Forexample, the current may be set at a maximum value and then modulate thepulse width. Or there may be coarse control of current by the clinician,setting it at approximately at the expected or estimated maximumrequired for that patient. While this current source can be implementedwith a current mirror, it is noted that keeping the current fixed at arelatively high current or maximum current, reduces the voltage dropacross the current mirror and therefore reduces the power dissipated inthe current mirror.

Device 1900 further comprises a pulse generator 1903 that is connectedto an electrode selector 1904 controlled by an electrode selectionsignal 1905 (set by the clinician). The electrode selector 1904 selectsfrom multiple electrodes a stimulation electrode 1906 to apply theelectrical energy to neural tissue 1907, return electrode 1908,measurement (sense) electrode 1909 and reference electrode 1910.

Device 1900 further comprises a differential amplifier 1911 thatamplifies the signal captured by sense electrode 1909 and provides thatto a correlator 1912 to calculate an ECAP amplitude 1913 as describedabove with reference to FIG. 5. In the example of FIG. 19, the ECAPamplitude is again the input to a feedback control circuit 1914, whichcalculates a pulse width 1915 provided to the pulse generator 1903. Inthis sense, the feedback control circuit 1914 increases the pulse widthto provide more stimulation energy when the ECAP amplitude is below adesired value and decreases the pulse width when the ECAP amplitude isabove a desired value. For example, a switched-mode converter with acurrent or voltage output may be configured to provide a fixed amplitudeoutput, with the pulse width being varied to provide the stimulusvariation needed for a closed loop stimulation system.

FIG. 20 illustrates the manner in which a stimulus 2001 produces an ECAP2002, which is then aligned with a template 1916 to provide amplitudemeasurement in the manner described in US20160287182 and with referenceto FIG. 5, which is included in its entirety herein by reference.

As nerve cells are mostly triggered throughout the duration of thecathodic phase of the stimulus pulse, when feedback control circuit 1914changes the pulse width 1915 provided to the pulse generator 1930, thetime between the start of the stimulus and the arrival of response atthe recording electrodes varies. The time of arrival of the ECAP 2002can be measured as the time to the arrival of the first peak of theECAP, the P1 peak, although other features may also be used. In orderfor the detector/correlator 1912 to work properly, the detector templateis aligned to be synchronous with the ECAP a during the detectionprocess.

One example of aligning the template 1916 involves a lookup table 1917which indicates the optimum delay between the stimulus and the detectionprocess for that particular pulse width. This optimum delay is then fedto a variable delay circuit 1918, which might be a variable-length shiftregister, to trigger the correlator, which determines the ECAP amplitudeas per US20160287182 and shown in FIG. 5 above.

As a result, the device 1900 generates stimulation current pulses andadjust the pulse length of the current pulses based on the measurednervous response of the tissue to reduce the dissipated power, while atthe same time aligning the template to the ECAP signal to accuratelymeasure the ECAP amplitude that is used for the feedback control thatultimately controls the width of the stimulation pulses.

In one example, the pulse width is controlled digitally by amicroprocessor. As a result, the pulse width has a limited number ofdifferent values, such as 256 different values for an 8-bit pulse widthsignal. In that case, the lookup table may have 256 different delayvalues, which is one delay value for each pulse width value. The delayvalues may be in the form of counter values for an internal processorcounter to reach the counter value before the template signal isgenerated. In other examples, the pulse width is continuous, such as afloat number or an analogue signal and the look-up table stores 256values. The variable delay 1918 module may then interpolate between theclosest values in the lookup table 1917 to determine the optimum delay.The look-up table 1917 may be replaced by a functional approximation ofthe relationship between the pulse width and the template delay, such asa linear function with two parameters or a polynomial with furtherparameters.

It is noted that the variable pulse width control described withreference to FIG. 19 may be combined with the switched-mode currentsource of FIG. 10 potentially including the circuits from FIGS. 13, 16and 17. In that case and referring back to FIG. 11, the resultingcircuit would control t₁ and t₃ as well as the pulse width PW to adjustthe stimulation level. However, in other examples, the circuit maintainst₁ and t₃ constant and only adjusts PW.

FIG. 21 illustrates a further example where the feedback control isimplemented directly in the converter, and avoids the use of asub-regulator. In particular, FIG. 21 illustrates an implantablestimulation device 2100 comprising a battery 2101, a switched-mode powersupply (SMPS) converter 2102, an H-bridge drive 2103 and an outputelectrode 2104 to supply an electrical stimulus to nervous tissue. Thereis also a sense electrode 2105 to capture an evoked response and afeedback circuit 2106. Importantly, the feedback circuit 2106 is nowconnected directly to the converter 2102.

When the converter is a voltage-to-current converter, the operation isas discussed above. If the converter is voltage-to-voltage, then this anintroduction to subsequent sections of this document.

The stimulator described below provide most of the benefits of voltagedrive, while fitting within the framework of existing clinical systemsand may be preferred amongst the variants mentioned in this disclosure.

FIG. 22 illustrates a further example stimulation device 2200 comprisinga battery 2201, a voltage-to-voltage SMPS converter 2202 connected to asub-regulator 2203 (similar to the current mirror in the previousfigures) and a smoothing capacitor 2204. The sub-regulator 2203 mirrorsa control current 2205 to an H-bridge drive 2206 which, in turn,delivers the current to a stimulation electrode output 2207. A sampleand hold circuit 2208 samples the stimulation voltage and a senseelectrode 2209 senses the evoked response, which is amplified by anamplifier 2210. The signals from amplifier 2210 and sample and holdcircuit 2208 are selectively switched by a switch 2211 onto ananalog-to-digital converter 2212 that converts the selected signal to adigital input of a digital controller 2213. The controller 2213 can thenuse the sampled stimulation output voltage and the sensed evokedresponse to calculate a control current of current source 2214 and acontrol voltage for converter 2215. Both are provided by respectivedigital-to-analog converters 2214, 2215. It is noted that the DACs 2214,2215 and ADC 2212 may be integrated into the digital controller 2213.

This design moves the SMPS control loop into digital controller 2213.The peak stimulus voltage is obtained by sample circuit 2208 samplingthe stimulus electrode at the end of the stimulus. This can be held inthe sample-hold circuit 2208, and converted to a digital value in theADC 2212. This ADC 2212 can be the same one used for digitizing thephysiological feedback. Since the digital controller 2213 will also beused to operate the physiologic control loop, it will have available thestimulus amplitude used for the next stimulus, and, via the digitalcontrol of the SMPS 2202, can prepare the power supply for the nextstimulus i.e. use feed-forward in the loop. The sample rate is typically60 Hz for many neuro-modulation applications, so the SMPS 2202 has morethan 10 ms to respond to the updated control voltage, making it asimpler design than that of FIGS. 7 and 8 which respond during thestimulus.

The digital controller 2213 can be a programmable microcontroller or adedicated state-machine.

FIG. 23 illustrates a voltage-drive version of FIG. 22. Again, there isa battery 2301, a voltage-to-voltage converter 2302, an H-bridge 2303connected to a stimulation electrode 2304. There is also a senseelectrode 2305 connected to an amplifier 2306 and a switch 2307 thatselectively connects the voltage from the converter 2302 or fromamplifier 2306 to an ADC 2308 connected to controller 2309. Thecontroller calculates the control voltage for converter 2302, which isprovided through DAC 2310 on an analog control voltage signal 2311.While the components shown in FIG. 23 are similar to those in FIG. 22,in FIG. 23 there is no sub-regulator 2202 as in FIG. 22. Instead, theconverter 2302 delivers the voltage directly to the stimulus output 2304(via H-bridge 2303). “Directly” in this context means without furthervoltage or current control since the H-bridge 2303 is only a connectioncircuit with no regulating function. In other words, the switches inH-bridge drive 2303 remain unchanged (on or off) during stimulation,while the switches in converter 2302 change many times to control thevoltage.

It is noted that most available converters that can be used as SMPS 2302have a feedback input to feedback the voltage at their output. Thisfeedback input is now used in FIG. 23 to provide the control voltagesignal 2311 to converter 2302. The main difference to the common usecase of closed-loop wired voltage feedback is that the feedback signal2311 is now generated by digital controller 2309. This way, controller2309 can control the converter 2302 by adjusting the voltage on controlsignal 2311. In one example, digital controller 2309 sets the controlsignal 2311 to a voltage level that is the desired voltage. However,there may be a difference, such as an offset between the desired outputvoltage and the control voltage 2311 if the converter 2302 includes ascale factor or offset due to varying battery voltage, for example.

With this implementation, controller 2309 can monitor the output voltageof the power supply to detect any over-voltage or to detect that despitethe maximum voltage has been applied there is little increase in thephysiological response. Further, controller 2309 receives the feedbacksignal from amplifier 2306 and can compare it to a desired value.Controller 2309 can then perform a control algorithm, such as PID, toreduce the difference between the measure and the desired evokedresponse by varying the output signal provided to DAC 2301. This mayresult in improved dynamic characteristics of converter 2302, such assettling time and ringing, compared to direct voltage feedback throughanalog comparison with a reference voltage. It also reduces the need foranalog components with are often a source of fluctuation and otherimplementation difficulties. It is noted that DAC 2310 may also beintegrated into converter 2302 in the sense that converter 2302 isconfigured to accept a digital signal and to control the output voltageaccordingly.

In one example, converter 2302 is a ringing choke converter comprisingprimary and secondary windings of a transformer and a base winding onthe primary side. A transistor is connected to the base winding so thata self-oscillation occurs in the primary side and at each oscillationthe transistor switches. This oscillation results in an induced currentin the secondary side, which can be smoothed by a transistor into a DCsignal. Importantly, the switching frequency depends on the inputvoltage and the state of the load. Now, the digital controller 2309controls that input voltage via voltage signal 2311 and therefore,controls the switching frequency in converter 2302. In turn, thiscontrols the output voltage of converter 2302 and finally thestimulation intensity on electrode 2304. As described herein, thecontroller 2309 adjusts the voltage on control signal 2311 based on thephysiological input.

FIG. 24 illustrates a further implementation of an implantablestimulation device 2400 where, compared to FIG. 23, the DAC 2310 isomitted and the controller 2309 directly provides the timed pulses forthe switching in switched-mode power supply converter 2302 through apulse width modulation output 2410. In particular, controller 2309generates arbitrary waveforms. It is noted that many microcontrollersalready provide the functionality of a pulse width modulation (PWM)control output. Therefore, processor 2404 may be a general purposemicrocontroller or other processor circuit, such as an FPGA or ASIC.

The duty cycle (ratio between ON and OFF), as defined by the PWM signal,defines the voltage of the output signal. This means that a change inthe conductance of the neural tissue is automatically compensated in thesense that a lower conductance leads to a higher voltage to achieve thesame evoked response.

More particularly, there may be two control loops. A first control loopcontrols the output voltage while a second control loop controls theevoked neural response. In one example, the first control loop isrepeatedly executed during the application of a stimulation pulse. Adesired output voltage is stored in digital controller 2309 and duringthe stimulation, the controller 2309 compares the present voltage(provided through switch 2307) to the desired voltage. If there is adifference, controller 2309 adjusts the duty cycle, such that the dutycycle is increased if the output voltage is less than the desiredvoltage and vice versa. The second control loop is executed once perstimulation phase, where the desired output voltage is adjusted based ona comparison between a desired evoked response and the measured evokedresponse. For example, controller 2309 increases the desired outputvoltage if the measured evoked response is below the desired evokedresponse and vice versa. Then, the desired voltage is used in the firstcontrol loop during the next stimulation phase. Known topologies forbuck-boost converters include SEPIC and Cúk topologies.

FIG. 29 illustrates a method 2900 for neural stimulation. The methodcomprising repeatedly performing the following steps in the sense thatthe following steps are repeated to provide continuous stimulation bymultiple stimulation pulses. So in one example, method 2900 is performedonce for each stimulation pulse.

The method 2900 commences by generating 2901 a stimulation voltagesignal at a stimulation voltage, such as by switching a switched-modepower supply. Next, the stimulation voltage signal is applied 2902 toneural tissue. A measurement circuit then measures 2903 a nervousresponse of the tissue. Finally, the stimulation voltage is adjusted2904 based on the measured nervous response.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the above-describedembodiments, without departing from the broad general scope of thepresent disclosure. The present embodiments are, therefore, to beconsidered in all respects as illustrative and not restrictive.

1. A device for applying a neural stimulus comprising: a battery tosupply electrical energy at a battery voltage; an electrode to apply theelectrical energy to neural tissue; a circuit to measure the nervousresponse of the tissue; and a voltage converter to receive theelectrical energy from the battery and to control a voltage applied tothe electrode based on the measured nervous response of the tissue. 2.The device of claim 1, wherein the converter comprises a processorprogrammed to calculate a voltage value based on the measured nervousresponse and to generate a control signal to the voltage converterindicative of the calculated voltage value.
 3. The device of claim 1wherein the converter circuit is a linear voltage-to-voltage converter.4. The device of claim 1 wherein the converter is a switched-modevoltage to voltage converter.
 5. The device of claim 4, wherein theconverter comprises a pulse generator configured to generate a pulsesignal to control switching of the switched-mode voltage to voltageconverter.
 6. The device of claim 5, wherein the pulse signal is basedon the measured nervous response of the tissue.
 7. The device of claim 4wherein the pulse generator is a digital processor.
 8. The device ofclaim 7, wherein the device comprises an analog-to-digital converter toprovide a digital signal indicative of the measured nervous response ofthe tissue to the digital processor.
 9. The device of claim 1 whereinthe voltage converter comprises a switch that is controlled by a controlsignal based on the measured nervous response of the tissue to therebycontrol the voltage applied to the electrode based on the measurednervous response of the tissue.
 10. The device of claim 9, wherein thecontrol signal defines a duty cycle based on the nervous response of thetissue, such that the control signal controls the switch and the dutycycle defines the output voltage to thereby control the voltage appliedto the electrode based on the measured nervous response of the tissue.11. The device of claim 9, wherein the control signal is an analogvoltage signal provided by the processor and the voltage signal controlsthe switching of the switch to thereby control the voltage applied tothe electrode based on the measured nervous response of the tissue. 12.The device of claim 11, wherein the controller comprises an oscillatorwith an oscillation frequency and the voltage signal controls theoscillation frequency to thereby control the voltage applied to theelectrode based on the measured nervous response of the tissue.
 13. Amethod for neural stimulation, the method comprising repeatedlyperforming the steps of: generating a stimulation voltage signal at astimulation voltage; applying the stimulation voltage signal to neuraltissue; measuring a nervous response of the tissue; and adjusting thestimulation voltage based on the measured nervous response.
 14. Themethod of claim 13, wherein generating the stimulation voltage comprisesrepeatedly switching a switched mode power supply; and adjusting thestimulation voltage comprises adjusting a pulse signal that controls theswitching.
 15. A device for applying a neural stimulus comprising: abattery to supply electrical energy at a battery voltage; an electrodeto apply the electrical energy to neural tissue; a circuit to measurethe nervous response of the tissue; a current mirror to deliver acurrent to the electrode according to a reference current that is basedon the measured nervous response; and a voltage converter to receive theelectrical energy from the battery and to control a voltage applied tothe current mirror based on a voltage between the stimulatingelectrodes.
 16. The device of claim 15, wherein the converter is aswitched-mode voltage converter.
 17. A device for applying a neuralstimulus comprising: a battery to supply electrical energy at a batteryvoltage; an electrode to apply the electrical energy to neural tissue; acircuit to measure the nervous response of the tissue; a switched modevoltage to current converter to receive the electrical energy from thebattery and to control a current applied to the stimulating electrode;and a controller to control switching of the switched mode voltageconverter based on the measured nervous response of the tissue.
 18. Thedevice of claim 17, wherein the controller is to control the switchingbased on the battery voltage.
 19. The device of claim 17 wherein thecontroller is to control the switching based on an electrode voltage 20.The device of claim 17 wherein the controller is to control theswitching based on a desired stimulation intensity.
 21. The device ofclaim 17 wherein the controller comprises a pulse generator to generatea pulse signal to control the switching.
 22. The device of claim 21,wherein the controller comprises a voltage controlled oscillator togenerate the pulse signal.
 23. The device of claim 17 wherein thecontroller comprises a voltage controlled delay controlled by thebattery voltage to control the switch.
 24. The device of claim 23wherein the voltage controlled delay is connected to a switch todisconnect an inductance from the battery after a delay controlled bythe battery voltage.
 25. The device of claim 23 wherein the voltagecontrolled delay is connected to the switch to disconnect the inductancefrom the battery after a delay controlled by a tissue voltage.
 26. Thedevice of claim 23 wherein the voltage controlled delay is connected tothe switch to disconnect the inductance from the battery after a delaycontrolled by a desired level of stimulation intensity.
 27. The deviceof claim 21 wherein the controller comprises a voltage controlledoscillator to control a frequency of the pulse signal based on a desiredlevel of stimulation and tissue voltage and a voltage controlled delayto control a time period for which the switch connects the inductance tothe battery at each oscillation based on the battery voltage.
 28. Thedevice of claim 21 wherein the pulse signal is periodic and controllingthe switch comprises suppressing pulses that turn the switch on to setthe amount of energy provided by the inductance.
 29. A device forapplying a neural stimulus comprising: a battery to supply electricalenergy at a battery voltage; an electrode to apply the electrical energyto neural tissue; a circuit to measure a nervous response of the neuraltissue; a pulse generator to generate stimulation current pulses at apulse length and to adjust the pulse length based on the measurednervous response of the neural tissue.
 30. The device of claim 29wherein the circuit to measure the nervous response of the neural tissuecomprises a template signal and the circuit is configured to shift thetemplate signal in time relative to the stimulation current pulses basedon the pulse width.
 31. The device of claim 30, wherein the circuitcomprises a look-up table storing delay values for the template signalfor each of multiple pulse width values.